1. Field of the Invention
This invention relates to the field of parking brakes for disk drives.
2. Art Background
Hard disk drives use aluminum disks to store blocks of data. The disks have magnetic surfaces and store data in blocks that lie along concentric circular tracks. The data is stored and recovered by read/write heads. These heads do not touch the surface of the disk, but ride on what is known as an "air bearing". Typically the heads are suspended 17-20 microinches from the surface of the disk.
The read/write heads are located on the end of a driver arm whose function is much like the tone-arm of a record player. However, since the heads of a disk drive do not touch the surface of the disk, an actuator motor provides an independent source of motion of the heads across the disk surface.
In order to optimize storage capacity, information is stored on both sides of a hard disk. Two heads are mounted on the drive arm, one for each side of the disk. The disk rotates at approximately 3000 rpm and contains a high density of storage tracks. For example, the present invention contemplates a hard disk having 600 tracks per inch.
Hard disk drives are typically used as storage devices for a host computer. When a read/write command is received from the computer, the drive arm is moved across the disk surface, seeking the appropriate track to access. The drive arm is propelled by the actuator motor.
The actuator motor consist of a fixed magnet attached to the drive housing, and a coil attached to the arm. Additional weight is added to the arm to counterbalance the arm, much like the counterweight on a record player tone arm. The driver arm is counterbalanced in order to equalize the moment of each side of the arm. The moment of the portion of arm extending on one side of the pivot point is dependant on the mass of the portion and the radius through which it acts. By counterbalancing the driver arm, the moments of each side are equal, and the arm can be operated in any plane without the need for complex servo circuitry. When the moment on each side of the pivot point is equal, the arm will not rotate unless additional force is applied to one side. Thus, changes in the plane of the arm, and shocks to the disk drive, will not cause the arm to rotate, reducing the chance of damage during shipping and other movement.
The rotation of the disk aids in maintaining the air bearing between the disk surface and the heads. When disk rotation stops, such as when the disk drive is turned off or suffers a loss of power, there is a risk that the heads may contact the disk. A head/disk contact is called a "crash" and may result in data loss or destruction of the disk medium. Additionally, when the disk drive is moved, shock may result in a head crash, again resulting in damage.
In order to prevent head crashes, or to reduce their effect, the prior art has utilized "parking tracks" and "parking brakes" on the disks and disk drives. A parking track is a track on a disk to which the heads are directed when the disk is not storing or retrieving data or when power is removed. The parking track contains no information and thus no data is lost in the event of a head crash. Similarly, should there be media damage to the parking track, there will be no reduction in the total storage capacity of the disk.
A parking brake is a device which is designed to retain the arm in a position where the heads will lie over the parking track. Prior art parking brakes have consisted of various spring mechanisms mounted on the arm assembly and activated by external controls. These devices have the disadvantage of adding to the weight, and therefore the inertia, of the driver arm. A second disadvantage is the large amount of current required by prior art parking brakes to hold them disengaged during operation of the driver arm.
As will be described, the present invention provides a parking brake mounted on the disk drive housing so as not to add to the weight of the drive arm. A coil mounted on the housing is used to hold the parking brake disengaged during operation of the disk drive. When the drive is turned off, the coil releases the brake and the brake, urged by a spring, pushes the arm to the parked position. When power is returned, the actuator motor is utilized to move the parking brake to a position where it can be held by the coil.